Why isnt the toupper working?

[SlenderMan]

I need it so even if you enter a capital Y instead of a lowercase y it just makes it all uppercase. Im trying to do this in line 36

  #include <iostream>
#include <string>
#include <conio.h>
#include <iomanip>
using namespace std;

int main()
{
	char response;
	int value = 0;
	char msg1[80] = "";
	char msg2[80] = "";

	do
	{
		cout << "Enter a phrase: ";			//get phrase one
		cin.getline(msg1, 80);

		cout << "\nEnter another phrase: ";	//get phrase two
		cin.getline(msg2, 80);

		cout << "\nPhrase \"" << msg1 << "\" has " << strlen(msg1) << " characters.";		//display amount of characters
		cout << "\n\nPhrase \"" << msg2 << "\" has " << strlen(msg2) << " characters.";

		value = strcmp(msg1, msg2);

		if(value == 0)
			cout << "\n\nIn C++, phrase \"" << msg1 << "\" is equal to the phrase \"" << msg2 << "\"";		//determine which phrase is greater
		else if(value > 0)
			cout << "\n\nIn C++, phrase \"" << msg1 << "\" is greater than the phrase \"" << msg2 << "\"";
		else if (value < 0)
			cout << "\n\nIn C++, phrase \"" << msg2 << "\" is greater than the phrase \"" << msg1 << "\"";
	
		cout << "\n\nPlay again(y/n)? ";
		cin >> response;
		toupper(response);
		cout << response;

		while(response != 'Y' || response == 'N')
		{
			if(response == 'N')
			{
				cout << "\n\nprogram ends.";
				break;
			}
			cout << "\nEnter a valid option: ";
			cin >> response;
			toupper(response);
		}	
	}
	while(response == 'Y');

	_getch();
	return 0;
}

[vlad from moscow]

response = toupper(response);